Currently, there are known a hammer drill and a diamond drill as concrete drills for boring a hole for mounting an anchor to a wall or the like constructed by concrete. A hammer drill subjects concrete to impact fracture to drill by rotating a drill bit attached with a sintered carbide tip at a front end thereof while exerting a striking force in an axial direction, a drilling speed is fast, prices of the bit and a concrete drill per se are inexpensive and therefore, a rate of spreading the hammer drill is high, however, there poses a problem that extremely large noise is emitted in operation.
The hammer drill rotates the drill bit and exerts an impulsive striking force and therefore, impact of the drill bit is directly propagated to a wall to vibrate the wall, vibration generated at a room at which operation is carried out is propagated in a wall or the like and propagated to a remote room as structural body propagating sound to emit large noise in a wide range. Therefore, there poses a problem that the hammer drill utilizing impact cannot be used in construction at a living site where living and sale are being carried on as in reforming or the like.
Meanwhile, according to a diamond drill, a front end of a bit in a cylindrical shape is attached with a tip embedded with diamond particles in a metal referred to as metal bond, and drilling is carried out by making the diamonds embedded in the tip at the front end bite concrete to polish by rotating the diamond bit. The size of a single particle of the diamond is about 400 micrometers, a single piece of the bit includes about 1500 particles of the diamond particles, the fine diamond particles cut concrete to drill and therefore, small sound is emitted, structural body propagating sound as in the hammer drill is not emitted, the sound is considerably low at a remote room to enable to carry out construction at a living site.
In the case of the diamond drill, in order to accelerate the driving speed, there is needed a predetermined pressing force for pressing the diamond tip to a concrete face. For example, in boring a large hole having a hole diameter of about 40 mm or larger, a fast drilling speed at low sound is realized by pressing the bit to concrete by fixing a tool of an installed type having a feeding mechanism to a wall face by a small-sized anchor and exerting a large pressing force by the feeding apparatus. Further, in boring a hole of up to about 12 mm, the diamond drill can sufficiently be reduced to practice even by a pressing force to a degree of capable of being pressed to a wall face by an operator although the drilling speed is slightly reduced.
However, the pressing force in pressing the concrete drill to the concrete face unforcibly by the operator is said to be limited to about 15 kgf in the horizontal direction and in the case of an anchor hole having a hole diameter of 16 through 38 mm, a large pressing force larger than 15 kgf is needed. In boring a hole about 16 through 38 mm, an operational time period is not so much prolonged and therefore, the installed type tool having the feed apparatus is not used but a hand-head tool is used, however, the larger the drilling diameter, the larger the contact area of the diamond tip and concrete and therefore, there poses a problem that the drilling speed is retarded since a sufficient pressing force is not exerted to the hand-head tool.